Many patients with prostate, ovarian, and other cancers have Circulating tumor cells, even in early-stage disease. Most studies show an association of circulating counts with disease stage, treatment response, and progression risk. Such counts are currently difficult to measure, as the 1-20 tumor cells/cc of blood are lost in a sea of 3-6 billion red blood cells. Flow cytometry -- ex vivo stained cells flowing past a fixed detector-- is an established technique which has allowed for detection (and sorting) of rare cells in solution. Here, we use cell low in the capillary bed to serve as an analogue of a flow chamber, to permit novel IN VIVO rare-cell counts. We propose to fluorescently label cells lowing in the capillaries, via i.v. injection of targeted dyes, and to use temporal spikes created by the physical flow of cells in the capillaries past a noninvasive optical detector to quantitate target cell count. In Phase I, we construct a fiber-based noninvasive cell detector, optionally including confocal imaging/spatial-filtering, to be placed over superficial capillary beds in tissues with thin mucosa, such as on the lip or sublingually. We test at least two injectable class-specific dyes: (1) PSMA-targeted to cells of prostate duct epithelial origin, and (2) folate-targeted to folate-receptor-overexpressing cancers. Because both the dyes and their endocytosing cell-surface targets are intravascular, delivery is rapid (secs) and binding is strong (100K/cell) at nM concentrations, yielding high cell-to-blood ratios. We test in a capillary model and rude mice. Hemoglobin at the same excite/emit wavelengths allows correction of counts for detected vascular volume. If successful, GMP dye synthesis and human testing begin in Phase II. In this revised Phase I (R43) study, specific aims are: (a) to produce a fiber-based probe, with or without confocal optics, for collecting light from subsurface capillary beds, (b) to demonstrate operation in a capillary bed model using PSMA-dye labeled LNCaP cells and folate-dye labeled KB cells, and (c) to demonstrate in nude mice with LNCaP and KB tumors, using both labeled and in-situ contrast injection. If successful, Phase II (R44) aims will be a) to develop GMP production of the agent; (b) to demonstrate safety of this agent in pre-clinical trials; (c) to apply for and obtain IND, IDE, and RB approval latest at least one agent, and (d) to test in women with/without ovarian cancer (and men with/without prostate cancer), to test performance in vivo. If successful, this may lead to improved staging, treatment monitoring, and possibly earlier diagnosis.